1. Field of the Invention
The present invention relates to the catalytic dehalogenation of alpha-halogenated carboxylic acids, and, more especially, to the selective catalytic dehalogenation of the dichloroacetic acid (DCAA) fraction present in monochloroacetic acid (MCAA).
2. Description of the Prior Art
The synthesis of monochloroacetic acid is carried out on an industrial scale by the chlorination of acetic acid, but dichloroacetic acid and sometimes a minor amount of trichloroacetic acid are also unavoidably formed. Therefore, upon completion of the chlorination reaction, a mixture of monochloroacetic acid, dichloroacetic acid, trichloroacetic acid and unreacted acetic acid is characteristically produced.
Because of the proximity of the boiling points of MCAA (189.degree. C.) and of DCAA (194.degree. C.), it is practically impossible to separate these species by distillation. On the other hand, it is quite simple to hydrogenate such a mixture to convert the DCAA into MCAA according to the reaction: EQU CHCl.sub.2 COOH+H.sub.2 .fwdarw.CH.sub.2 ClCOOH+HCl
But this hydrogenation is not completely selective and a reversion of MCAA to acetic acid is also observed, as follows: EQU CH.sub.2 ClCOOH+H.sub.2 .fwdarw.CH.sub.3 COOH+HCl
This reaction is carried out in the presence of a catalyst and also produces acetaldehyde, which has the disadvantage of giving rise to objectionable condensation products.
FR 1,581,391 describes such a process based on a catalyst of silica in the form of cylinders 8 mm in length and 3.5 mm in diameter, which has a palladium content of 0.5% by weight.
FR 2,039,987 describes a process of the same type as the above, but entailing introduction of either salts or oxides of the alkali or alkaline earth metals or of rare earths, or an organic compound designated a proton-acceptor, which may be, for example, triethylamine, pyridine, piperidine, glycine, urea, triphenylphosphine, butyraldehyde, diisobutyl ketone, butyl acetate or hydrocyanic acid. This catalyst activator is introduced into the mixture to be hydrogenated and can be recovered by distillation of the hydrogenated mixture. It can be recycled into the mixture to be hydrogenated.